where the adjectives outside the brackets mean simply what we have already stated them to mean. When once the initial attitude has been taken, and the world to be explored has thus been determined, the methods are the same. The beginner in psychology will however find, again and again, that his common-sense self stands in the way of disinterested observation; and as the word introspection contains a reference to this self, he may prefer to drop it altogether. So much for observation in general! When we come to particulars, we find that science, wherever possible, has recourse to =experiment=. This does not mean that science renounces observation. For an experiment, if we push our definition back to fundamentals, is simply _an observation that may be repeated, that may be isolated, and that may be varied_. See the advantages! Repetition gives us plenty of time for observation; we need not mind overlooking something now, since we shall have the opportunity of picking it up later; and we can go on, observing and observing, until our description of the phenomenon is as complete as it can be made. Isolation makes our task easier; disturbing influences are ruled out; our attention is not distracted; we can give ourselves wholly to the matter in hand. Variation—the substituting of one factor for another in successive observations, or the emphasising in one observation of a factor that was obscure in another—helps us to clear up doubtful points; to distinguish what is universal from what is only accidental in the phenomenon we are observing; and to bring this phenomenon into relation with kindred phenomena. _Repetition saves hurry and worry; isolation prevents distraction; variation keeps us from jumping at conclusions._ These are the advantages of experiment; and all experiments, in physics, in chemistry, in biology, everywhere, fall under this definition.